The following filters are known:
filters having circular or rectangular (waveguide) cavities and a high Q factor, e.g. greater than 3000; PA1 cylindrical or rectangular coaxial filters of the end coupling type (of wavelength .ltoreq..lambda./2; where .lambda. is the wavelength of the guided wave) or of the coupled line type (wavelength .ltoreq..lambda./4) having a Q factor of not more than 1000; PA1 microstrip filters on dielectric substrates but having very low Q, below 200, together with non-negligible insertion losses; and PA1 dielectric resonator filters having an intermediate Q lying between 100 and 3000. PA1 good reproducibility due to the fact that a chemical photo etching technique is used; PA1 low cost due to the simplicity of the circuit; and PA1 no adjustment necessary for using the filter.
However, implementation of such filters at high frequencies remains both difficult and expensive.
Another possibility currently in use consists in using end coupled .lambda./2 resonant linesmounted on a suspended substrate.
An article entitled "Design and performance of millimeter wave end coupled bandpass filters" published in "International Journal of Infrared and Millimeter Waves" (Volume 6 No. 7 1985) describes filters of this type in which the resonant lines are formed by sequences of periodic discontinuities situated along transmission lines in order to form series of resonators which are coupled to one another.
The invention seeks to provide an apparatus having the advantages of this type of filter, namely:
However, the invention also makes it possible to modify the passband and it makes it possible to integrate a stop band function which allows absorption or rejection of waves.
An apparatus in accordance with the invention is capable of operating in a frequency range running from 1 GHz to 100 GHz.